The problem of measuring the flow-rates of multi-phase fluids in a pipe without the need to interrupt fluid flow or separate the phases during the measurement process is of particular importance in the chemical and petroleum industries. Because almost all wells produce a mixture of oil, water, and gas, flow measurements of the individual components of the fluid mixture are essential in the efficient production of a reservoir. Conventionally, at the surface, these measurements were made through separators, which are costly and bulky, especially for offshore applications.
The above problem has been addressed by multi-phase flow-meter devices which are now commonly used in the oil and gas industry and other chemical industries. Such devices measure flow velocity of various components of a multi-phase fluid mixture by measurement of Gamma ray or X-ray attenuation through the mixture at two different energy levels, namely, a “high” energy level and a “low” energy level. The measurements are based on the fact that the absorption coefficient of the Gamma ray/X-ray radiation is dependent on the material and the photon energy. Accordingly, the “high” energy level is determined such the photon absorption coefficient at this energy level of photons is substantially the same for oil and water. The “low” energy level is determined such the photon absorption coefficient at this energy level of photons is significantly higher for water than for oil. The Gamma rays/X-rays pass through the mixture in a test section of the pipe and irradiate detectors that are sensitive to photons at these two energy levels. Analysis of the signals recorded by the detectors allows evaluation of water, oil and gas flow-rates passing though the test section.
The volumetric flow-rate calculations in such prior art devices are based on pressure differential measurements, for which reason, the test section is provided with a contraction, such as a Venturi restriction. A Venturi restriction interferes with the fluid flow. Further, such an arrangement provides limited precision in flow measurements and is particularly disadvantageous in case of non-uniform flow composition, in particular, across the cross-section of the mixture flow.